在Java中使用simpledateformat增加n天

Question

We have a java code snippet here

import java.text.SimpleDateFormat;
import java.util.Date;
public class SimpleDateFormatExample {
public static void main(String[] args) {
    Date date = new Date();
    int days = 5;
    SimpleDateFormat formatter = new SimpleDateFormat("dd/MM/yyyy");
    String strDate= formatter.format(date.getTime() + (days*86400000));
    System.out.println(strDate);
}
}

to add n no. of days to today's date. The result will be correct upto n=24 but gives previous month' after n=24 . Why it is so?

Answer 1

The problem is the the int is overflowing

consider

    int days = 25;
    int d = days*86400000;
    System.out.println(d);

try

    int days = 25;
    long d = days*86400000L;
    System.out.println(d);

Answer 2

tl;dr

LocalDate               // Represent a date-only, without a time-of-day and without a time zone.
.now()                  // Capture the current date, as seen through your JVM’s current default time zone. Better to pass a `ZoneId` as the optional argument.
.plusDays( 5 )          // Add five days, returning a new `LocalDate` object. Per the Immutable Objects pattern, a new object is produced rather than changing (“mutating”) the original.
.format(                // Generate text representing the date value of our `LocalDate` object.
    DateTimeFormatter.ofPattern( "dd/MM/uuuu" ) // Define a formatting pattern to suit your taste. Or call the `.ofLocalized…` methods to localize automatically. 
)                       // Returns a `String`.

java.time

Date class represents a moment in UTC, a date with a time-of-day, and an offset-from-UTC of zero. Wrong class to use when working with date-only values.

Avoid using the terrible old legacy date-time classes such as Calendar , Date , and SimpleDateFormat . These classes were supplanted years ago by the java.time classes.

Do not track days as a count of seconds or milliseconds. Days are not always 24 hours long, and years are not always 365 days long.

LocalDate

Instead, use LocalDate class.

LocalDate today = LocalDate.now() ;
LocalDate later = today.plusDays( 5 ) ;

Convert

Best to avoid the legacy classes altogether. But if you must interoperate with old code not yet updated to java.time classes, you can convert back-and-forth. Call new methods added to the old classes.

For Date you need to add a time-of-day. I expect you will want to go with the first moment of the day. And I'll assume you want to frame the date as UTC rather than a time zone. We must go through a OffsetDateTime object to add the time-of-day and offset. For the offset, we use the constant ZoneOffset.UTC . Then we extract the more basic Instant class object to convert to a java.util.Date .

OffsetDateTime odt = OffsetDateTime.of( later , LocalTime.MIN , ZoneOffset.UTC ) ;  // Combine the date with time-of-day and with an offset-from-UTC.
Instant instant = odt.toInstant() ;  // Convert to the more basic `Instant` class, a moment in UTC, always UTC by definition.
java.util.Date d = java.util.Date.from( instant ) ;  // Convert from modern class to legacy class.

Going the other direction:

Instant instant = d.toInstant() ;  // Convert from legacy class to modern class. 

---

About java.time

The java.time framework is built into Java 8 and later. These classes supplant the troublesome old legacy date-time classes such as java.util.Date , Calendar , & SimpleDateFormat .

The Joda-Time project, now in maintenance mode , advises migration to the java.time classes.

To learn more, see the Oracle Tutorial . And search Stack Overflow for many examples and explanations. Specification is JSR 310 .

You may exchange java.time objects directly with your database. Use a JDBC driver compliant with JDBC 4.2 or later. No need for strings, no need for java.sql.* classes.

Where to obtain the java.time classes?

• Java SE 8 , Java SE 9 , Java SE 10 , Java SE 11 , and later - Part of the standard Java API with a bundled implementation.
  • Java 9 adds some minor features and fixes.
• Java SE 6 and Java SE 7
  • Most of the java.time functionality is back-ported to Java 6 & 7 in ThreeTen-Backport .
• Android
  • Later versions of Android bundle implementations of the java.time classes.
  • For earlier Android (<26), the ThreeTenABP project adapts ThreeTen-Backport (mentioned above). See How to use ThreeTenABP… .

The ThreeTen-Extra project extends java.time with additional classes. This project is a proving ground for possible future additions to java.time. You may find some useful classes here such as Interval , YearWeek , YearQuarter , and more .

Answer 3

使用days*86400000L进行long计算，否则int值将溢出。

Answer 4

Try this one in your code:

Calendar cal = Calendar.getInstance();
cal.setTime(new Date());
cal.add(Calendar.DATE, 5); 

strDate = formatter.format(cal.getTime());